Weather Eye with John Maunder

Global (or eustatic) sea level change is measured relative to an idealised reference level, the geoid, which is a mathematical model of planet Earth's surface.

Global sea level is a function of the volume of the ocean basins and the volume of water they contain.

Changes in global sea level are caused by – but not limited to - four main mechanisms: (Source: www.climat4you.com)

1. Changes in local and regional air pressure and wind, and tidal changes introduced by the Moon.

2. Changes in ocean basin volume by tectonic (geological) forces.

3. Changes in ocean water density caused by variations in currents, water temperature and salinity.

4. Changes in the volume of water caused by changes in the mass balance of terrestrial glaciers.

In addition to these there are other mechanisms influencing sea level; such as storage of groundwater, storage in lakes and rivers, evaporation, etc.

Mechanism 1 is controlling sea level at many sites on a time scale from months to several years.

As an example, many coastal stations show a pronounced annual variation reflecting seasonal changes in air pressures and wind speed.

Longer-term climatic changes playing out over decades or centuries will also affect measurements of sea level changes.

Mechanism 2, with the important exception of earthquakes and tsunamis, typically operates over long (geological) time scales, and is not significant on human time scales. It may relate to variations in the sea floor spreading rate, causing volume changes in mid-ocean mountain ridges, and to the slowly changing configuration of land and oceans.

Mechanism 3 (temperature-driven expansion) only affects the uppermost part of the oceans on human time scales.

Usually, temperature-driven changes in density are more important than salinity driven changes.

Seawater is characterised by a relatively small coefficient of expansion, but the effect should however not be overlooked, especially when interpreting satellite altimetry data.

Temperature-driven expansion of a column of seawater will not affect the total mass of water within the column considered and will therefore not affect the potential at the top of the water column.

Temperature-driven ocean water expansion will therefore not in itself lead to lateral displacement of water, but only lift the ocean surface locally.

Mechanism 4 (changes in glacier mass balance) is an important driver for global sea level changes along coasts, for human time scales.

Volume changes of floating glaciers – ice shelves – has no influence on the global sea level, just like volume changes of floating sea ice has no influence.

Only the mass-balance of grounded or land-based glaciers is important for the global sea level along coasts.

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For further Infomation about a wide range of weather/climate matters see my new book "Fifteen shades of climate... the fall of the weather dice and the butterfly effect".

The extract above on "sea level changes" is from pages 220-221 of my book.